We
cannot be certain where or when life began. It was a long
time ago, but good detective work by earth scientists, astronomers,
astrophysicists and microbiologists,
amongst others, has gathered a lot of evidence that gives us some
clues. These clues suggest five possibilities:

On
Earth's surface, in warm oceansInside
the Earth, in the rocks of the mantleOn
another planet or moonIn
deep spaceIn
many places, independently

But
to understand the meaning of these clues there is something else
we will need to think about, too.

If
life started on Earth,
then we know that it has been here for around 3.6 billion years,
and possibly as long as 3.85 billion years. Micro-fossil bacteria
have been found in some of the oldest rocks in the world, and they
date from the later period. Some rocks, however, suggest that
life is even older. It may have started just 500 million
years after the Earth
formed. During this primordial period the planet
was undergoing severe bombardment
by meteorites. Enormous chunks of rock and metal, the
size of countries, were crashing into the surface of Earth at speeds
as high as 35 kilometers per second (around 80,000 mph). Craters
the size of Southern California, or even bigger, were formed and
the energy release would have been enough to raise the surface temperature
of the planet to a sustained 3,000 C (nearly 5,500 F). Additionally,
at some point during this era, a planet the size of Mars collided
with the proto-Earth and a double body - Earth
and Moon - were formed.

The
immense surface temperatures must have precluded the presence
of liquid water. Gradually though, from around 3.8 billion
years ago, the planet cooled to a point where steam and water
vapor were able to condense out of the atmosphere which, at that
time, was rich in carbon-dioxide and low in oxygen. Oceans
formed, but the origin of the Earth's water is uncertain.
It may have come from the original materials from which the planet
formed, or it may have arrived later through the visitation of
comets, which are largely water ice.

Until
recently it was speculated that the first life on Earth -
probably simple bacteria - developed in the oceans, in the liquid
water medium, where it was able to flourish. This idea originated
from Charles
Darwin who, in 1871, wrote that he imagined life originating
in "... some warm little pond, with
all sorts of ammonia and phosphoric salts, light, heat, electricity
etc...". The idea, from such an eminent person
as Darwin, was largely unquestioned for a long time and led to the
theory being tested by the Miller-Urey experiment
of the 1950's.

The
complex chemicals necessary for the formation
of amino acids, and ultimately DNA,
may have been created by a combination of the nutrient-rich ejecta
- dust, gas and lava - of volcanic vents in the sea (ie in chemical
terms - sulfur, nitrogen, calcium and phosphorus for instance) and
weather energy, such as lightning and solar radiation. If
this was the process, then the complex molecules of life must have
been created spontaneously from these precursors. The exact
process was and remains unknown. It has never been replicated
in a laboratory, though some researchers
claim to be close to doing it.

It
has been suggested by researchers, such as physicist Dr
Paul Davies of the University of Adelaide, that it would have
been much easier for life to start on early Mars than it would have
been on early Earth. Large meteorite impacts on
Mars could have thrown living material into space, where it could
have been carried to Earth inside small meteorites. Once here,
it found favorable conditions and flourished.

While
Earth and Mars are suitable candidates
for this process, no other large members of the Solar
System - the Giant Gas
and Ice planets,
for instance - seem to offer the same conditions, so it is
unlikely to have occurred. However, smaller objects such as
Europa in space may offer something...

There
is growing evidence that very simple life may occur in deep space,
and be riding on and inside comets. If so, the Earth may have
been seeded many times with biological matter from space.
Life itself may have originated, not on Earth but in deep interstellar
space, five billion or more years ago.

If
this is so, it may well have come into contact with life from an
alien origin and it may even have exchanged DNA.
For this to have happened though, many obviously coincidental things
must have happened, which will not be elaborated here.
Instead, ponder one of the most important questions that all this
raises:

Does
the life-creation process - whatever that is - always work in
the same way?

These
questions cannot be answered, and may never be answered. Most
microbiologists and other scientists would maintain that life, as
we know it, is still only to be found on planet Earth. This
is despite compelling evidence from martian meteorites, experiments
carried out by spacecraft, and fundamental research into deep space
habitats.

The
questions are still open, but let us take a look at some clues
by taking a look at: